The present invention relates to an anchoring arrangement in general, and more particularly to a rock anchor for use in tunnels, mines or similar constructions, especially such which are subjected to considerable convergences.
Anchor rods have been known, provided with a circumferential projection or ridge which has been formed on the leading end of the anchor rod which has been inserted into a predrilled borehole. An adhesive has been used to fix the leading end of the anchor within the borehole. The end portion of the anchor rod, extended outwardly from the borehole is provided with the fastening thread, the latter extending centrally through a dish-like supporting plate which is supported against the face of a working. A nut screwed onto the fastening thread of the anchor rod presses the supporting plate circumferentially against the surface around the open end of the borehole.
If convergences in the rock exist all the forces exerted on the anchor rod act exclusively on the supporting plate. The result of these forces acting directly on the supporting plate is the occurrence of exfoliations in the rock strata in the region of the open end of the borehole. It has been suggested in practice to use the construction which is glued by adhesive in the borehole over the entire length thereof. However, in this instance forces occurring in the event of convergences in the rock strata are translated only to the supporting plate. In order to provide that the intermediate longitudinal portion of the anchor rod would also take up the rock strata forces anchor rods have been utilized, which have been provided over their entire length with circumferential projections. Concrete-reinforced steel has been usually used for the anchor rods. Such anchors have been fixed in the boreholes over their entire length with an adhesive. With such a construction, the movements of the rock strata have been transmitted onto the rock anchor immediately at the area of their formation and not only onto the supporting plate. It has been found that in the event of substantial convergences local overloads occur in the anchor rods, which lead to cracking of the rods at the locations of these overloads.